1. Field of the Invention
The present invention relates to a semiconductor device including a bipolar transistor, and more particularly, it relates to a semiconductor device including a high concentration buried layer formed by high energy ion implantation, and a method of fabricating the same.
2. Description of the Background Art
In general, an integrated circuit including a bipolar transistor is provided with a buried impurity layer for serving as a floating collector.
FIG. 13 is a sectional view showing a conventional semiconductor device including a bipolar transistor. An epitaxial layer 11 of several .mu.m in thickness is provided on a buried impurity layer 2, and a bipolar transistor 100 which is isolated by an element isolation film 6 and an isolation region 7 is formed on the epitaxial layer 11. The epitaxial layer 11 serves as a collector region of the bipolar transistor 100, which is formed by an emitter region 5 and a base region 4.
Due to employment of the epitaxial layer 11, however, the conventional semiconductor device requires long-time epitaxial treatment in fabrication steps thereof, and an apparatus required for such epitaxial treatment leads to increase in cost.
Further, the element isolation film 6 which is formed by thermally oxidizing the epitaxial layer 11 is inevitably increased in thickness.
On the other hand, the buried impurity layer 2 is formed before formation of the epitaxial layer 11, and hence an automatic doping effect is caused by an impurity contained in the buried impurity layer 2 through heat treatment for epitaxial growth and an oxidizing step for forming the element isolation film 6. Further, the buried impurity layer 2 expands during formation of the epitaxial layer 11 to erode the same, while the degree of such erosion is varied with the area of the buried impurity layer 2 in a loading effect. Such automatic doping and loading effects make it difficult to control formation of the buried impurity layer 2, leading to difficulty in characteristic control of the transistor 100 provided thereon.
In a general bipolar transistor, further, a leakage current results from internal defects thereof or contamination by heavy metals.